From: Mobile and disconnected computing

System software for mobile computing is impeded by four distinct
constraints:

- Compared to stationary computers, mobile computers will always be
  resource poor [Satyanarayan, 93].  Although currently available PDAs
  (Personal Digital Assistants) compare favourably with the
  stand-alone workstations of a few years ago [Marsh, 93], they'll
  most probably lag behind in compute capabilities, available power,
  storage availability and communication bandwidth, for some time to
  come.

- Mobility entails computation amid fluctuating resource availability
  and constraints [Banerji, 93].  Communication bandwidth may be
  available at discrete intervals, an available resource may suddenly
  become unreachable or an otherwise in-expensive communication link
  may be randomly replaced by an expensive alternate in transit.

- Security threats to both mobile computational elements as well as
  the data accessed by them are greatly increased [Satyanarayan, 93].
  Not only is it easier to lose, damage or be robbed of a carry-along
  PDA, but it is often easier to tap into the data transferred (as is
  well-known to much of the cellular communication industry).  Very
  little work, except for that undertaken by the cellular
  communication industry, has been done in the area of addressing the
  specific security needs of mobile computing (as far as I know).

- User needs and their application requirements may not be the same as
  those in stationary systems [Weiser, 91].  As mobile computers
  become ubiquitous (this phrase coined by Mark Weiser), the number of
  computer users will most probably increase exponentially.  Most or
  many of these users will be far less computer literate than the
  average computer user of today.  In addition, shopping, information
  browsing and entertainment may be the typical use of such mobile
  units, as opposed to traditional scientific computing, database
  support or word processing.

- With the presence of PCMCIA slots in a PDA, it also becomes
  necessary for an OS to be able to mount and dismount entire OS
  subsystems on the fly [Hildebrand, 94].  Operating systems need to
  be able to treat networking, filesystems, and other services as
  facilities which may be loaded and unloaded on demand.
  
Based upon an amalgam of these criteria, the next few sections discuss
some of the main areas of ongoing research in mobile computing.